The departing fund manager, who joined the company in 1993, was responsible for running a number of investment vehicles including Irish funds and smaller companies funds at the firm.
Adam McConkey, a senior investment manager within the team with 11 years worth of experience in the industry is to take over from Mr Williams when he leaves the firm in September.
Dominic Rossi, chief investment officer at Gartmore, said: "It is testament to the strength in depth in the Gartmore UK Smaller Companies team that we can call upon a manager of Adam's calibre.
âWe are confident that, with the support of the hugely experienced Rob Giles and the rest of the team, Gartmore will continue to be a leading light in the specialist management of UK Small Cap equities.â
He added that the firm would like to thank the departing post holder for his âcontributionâ.
The departure of Mr Williams follows the resignation of high profile fund manager Guillaume Rambourg earlier in the year, following an ongoing investigation into his trading activity by the Financial Services Authority.
By Jim Ottewill